Taking temperatures of nuclear transitions.

Most earthly matter dwells in a sleepy neighborhood that has been largely neglected by nuclear physics. It's the burg populated by unexcited nuclei that appear to be rich in orderliness but poor in energy.

In recent years, Norwegian physicists have devised a technique to probe the structure of nuclei inhabiting such low-energy landscapes. In an upcoming PHYSICAL REVIEW LETTERS, Elin Melby and her colleagues at the University of Oslo will report extending the technique to allow them for the first time to infer the temperature of such nuclei as they go through some intriguing transformations.

The researchers deduce temperature from the way the neutrons and protons, together called nucleons, rearrange themselves when energy is added to the nucleus. Knowing these temperatures will make it possible to better identify the stages that nuclei pass through as they crumble from an orderly arrangement of nucleons into a disorderly high-energy gas, says Magne Guttormsen, who led the research team.

Until an unexcited nucleus gains about 2 million electron volts (MeV) of energy, theories predict, its nucleons should orbit in well-defined pairs of identical particles. Each nucleon pair shares one internal energy level. In line with these predictions, the data gathered by the Norwegian team indicate that the pairs start to break up as the overall nuclear energy increases to more than 2 MeV.

"We are just now trying to find out the critical temperature," Guttormsen says.

A succession of jumps in the number of energy levels that the nucleons can occupy also indicates that the pairs split apart one at a time, not in a rout, he adds.

So far, the scientists have determined an average temperature of one type of nucleus as the first few nucleon pairs break up: roughly 6 billion kelvins, or 375 times as hot as the center of the sun. "It's actually a quite cold system," says Guttormsen, compared with extremely excited nuclei at much higher temperatures in accelerator experiments (SN: 4/15/95, p. 228).

Exciting low-energy nuclei to study them without destroying their internal structure has proven challenging. The Norwegians have explored the transitions by bombarding the heavy nuclei of the rare earth elements dysprosium, erbium, and ytterbium with light, relatively low-energy helium-3 ions. The ions steal neutrons from the target, changing themselves into helium-4, and leave behind nuclei excited just enough to give off gamma rays that lay bare their inner workings.

By learning more about such transitions, researchers may deepen understanding not only of nuclear structure but also of superconductivity and the behavior of tiny objects composed of only a few atoms, comments Teng L. Khoo of Argonne (Ill.) National Laboratory.

Most striking, Khoo finds, is the Norway team's evidence of steps. Compared with transitions of electron pairs in super-conducting metals, which suddenly fall apart at certain temperatures, the rare earth transitions are "smeared," he says.

Although previous calculations have shown that pair disintegrations take place progressively, "we've never been able to trace how that happens," Khoo says. "This paper is the first good glimpse of this stepwise smearing of this transition."

Steven Koonin of the California Institute of Technology in Pasadena says that he and his colleagues are planning further calculations in the same low-energy range to see if they also can find a step pattern in the transition. "It's difficult to get down to such low energies with good resolution for us. With more computers and computer time, we're going to try that," he says.